Grits such as crushed corundum contain, after crushing grinding and grading, a more or less great percentage of iron, which is undesirable in the manufacture of grinding wheels, for example, or other ceramically bonded products. This iron content is partially removed magnetically from coarse grits. In the case of fine grits, however, magnetic iron removal is not possible.
The iron is then removed by chemical methods, using for example 20% hydrochloric acid in a batch-wise procedure. In general, two known processes are used, which can be explained as follows using corundum as an example:
1. Removal of iron by washing in boiling hydrochloric acid. PA1 2. Removal of iron with cold hydrochloric acid or hydrochloric acid warmed to about 60.degree. C.
In this process, 18 wt-% hydrochloric acid is brought to a boil in enameled, steam-heated kettles. The corundum is sprinkled into the boiling acid. The iron removing process takes two hours. Then the corundum-and-acid mixture is dumped into a settling tank made of plastic. After 7 to 8 washings with fresh water and decantations, the acid is removed. The washed corundum is then poured into drying hurdles and dried in steam-heated or gas-heated circulating air dryers.
This method of iron removal by boiling hydrochloric acid is very effective, but it requires a great deal of hand labor and is very time consuming and expensive. In addition, it is very difficult to neutralize all of the vapors of the boiling acid. Another problem occurs when the acid is poured off or decanted, in that from time to time severe acid surges occur which are technically very difficult to neutralize. Both the neutralization of the vapors and the discharge of the acid into the sewers create great problems for the environment.
In this process the corundum is added to cold or slightly heated hydrochloric acid in plastic containers. The washing process is performed with an electrically powered agitator. The washing with water is performed in the same vessel by repeated stirring, followed by settling, and repeated changes of the water.
At the end of the washing process the corundum must be removed by pumping or by hand from the washing tanks and placed on drying hurdles and then further dried in drying ovens. This process, too, requires much hand labor.
The need which according exists for a continuously performable process for the removal of iron from grits is made difficult to satisfy by the fact that severe erosion is produced in all equipment that comes in contact with this material owing to its abrasiveness caused by its hardness and the manner in which it is ground.